Fixes tortoise-orm#1987

1. Use Index.describe() to encode Index object instead of pickle for tortoise-orm>=0.24
2. Only use pickle to load obj['val'] when obj.get('type') == 'index' for object_hook
3. Add index_type and extra to ddl._ADD_INDEX_TEMPLATE
4. Add IF EXISTS into ddl. _DROP_INDEX_TEMPLATE
5. Add IF NOT EXISTS into postgres index create template
6. Use load_index to convert dict to Index instance when comparing indexes
7. New test case:

• Index --> HashIndex for PostgreSQL
• Index --> FullTextIndex for MySQL